Hi GGPatrol
If you give me your location i can tell you a few suppliers
sas has a special this week on composite panel
but composite only comes in 3mm and 4mm
The product i would recomend is foam board comes in 3/4/6/8/10/12/16/25/30mm
in lots of sheet sizes
screws together like ply and can be glued with pvc glue or silicone
it is really lightweight. But different companies call it different names
I think your in melbourne ? so try a company ca,,ed plastral fidene or mulford plastics
if you have a problem pm me